Acute graft-versus-host disease (GVHD) considerably limits wider usage of allogeneic hematopoietic cell transplantation (allo-HCT). Antigen-presenting cells and T cells are populations customarily associated with GVHD pathogenesis. Of note, neutrophils are the largest human white blood cell population. The cells cleave chemokines and produce reactive oxygen species, thereby promoting T cell activation. Therefore, during an allogeneic immune response, neutrophils could amplify tissue damage caused by conditioning regimens. We analyzed neutrophil infiltration of the mouse ileum after allo-HCT by in vivo myeloperoxidase imaging and found that infiltration levels were dependent on the local microbial flora and were not detectable under germ-free conditions. Physical or genetic depletion of neutrophils reduced GVHD-related mortality. The contribution of neutrophils to GVHD severity required reactive oxygen species (ROS) because selective Cybb (encoding cytochrome b-245, beta polypeptide, also known as NOX2) deficiency in neutrophils impairing ROS production led to lower levels of tissue damage, GVHD-related mortality and effector phenotype T cells. Enhanced survival of Bcl-xL transgenic neutrophils increased GVHD severity. In contrast, when we transferred neutrophils lacking Toll-like receptor-2 (TLR2), TLR3, TLR4, TLR7 and TLR9, which are normally less strongly activated by translocating bacteria, into wild-type C57BL/6 mice, GVHD severity was reduced. In humans, severity of intestinal GVHD strongly correlated with levels of neutrophils present in GVHD lesions. This study describes a new potential role for neutrophils in the pathogenesis of GVHD in both mice and humans.

Key Points Our data from the mouse model and patients indicate that inflammatory neovascularization during GvHD is targetable via αv integrin. We identify a negative regulation of GvHD-related neovascularization by miR-100.

During the course of atherosclerotic vascular disease, the adaptive growth of blood vessels is a naturally occurring process that can partly compensate for the decrease in blood flow after the narrowing or occlusion of a major artery. It includes both the sprouting of new endothelial capillaries (angiogenesis) and the enlargement of pre-existing arteriolar and arterial anastomoses to functional collateral arteries (arteriogenesis).1 During angiogenesis, a drop in tissue oxygen tension results in increased expression of hypoxiainducible transcription factors and cytokines, stimulating endothelial proliferation and sprouting in the ischemic tissue, improving distribution and use of the remaining blood flow. On the other hand, arteriogenesis is characterized by a well-orchestrated inflammatory response that is not restricted to the endothelial cell (EC) layer but facilitated by the perivascular infiltration of bone marrow-derived cell populations, mediating the proliferation of both endothelial and vascular smooth muscle cells. During the past decade, monocytes and macrophages were especially demonstrated to exert an important stimulatory function in the regulation of collateral artery growth.2 Although our knowledge about these contributing cell populations in the different forms of vascular growth steadily increases, our understanding of the basic regulatory principles controlling these processes is still limited. Other than canonical mediators of blood vessel growth, such as growth factors and their receptors, an additional functional group of regulators has recently emerged: microRNAs (miRNAs). These short (17-24 nucleotides), single-stranded regulatory RNA sequences are transcribed as precursor hairpin structures from intergenic or intronic regions of the genome that undergo several nuclear and cytoplasmatic processing steps to the mature miRNA.3 Together with Argonaute proteins, they form the RNA-induced silencing complex and recognize specific sequences mostly located in the 3′ untranslated region of their target mRNA, resulting either in inhibition of translation or degradation of Background-Adaptive neovascularization after arterial occlusion is an important compensatory mechanism in cardiovascular disease and includes both the remodeling of pre-existing vessels to collateral arteries (arteriogenesis) and angiogenic capillary growth. We now aimed to identify regulatory microRNAs involved in the modulation of neovascularization after femoral artery occlusion in mice. Methods and Results-Using microRNA-transcriptome analysis, we identified miR-155 as a downregulated microRNA during hindlimb ischemia. Correspondingly, inhibition of miR-155 in endothelial cells had a stimulatory effect on proliferation and angiogenic tube formation via derepression of its direct target gene angiotensin II type 1 receptor. Surprisingly, miR-155-deficient mice showed an unexpected phenotype in vivo, with a strong reduction of blood flow recovery after femoral artery ligation (arteriogenesis) dependent on the attenuation of leuko...